1. Field of the Invention
The present invention relates to a malfunction detection system for securing the safe operation of a transfer device mounted on a transfer press.
2. Prior Arts
A transfer press is a type of forging press for efficiently handling with plural workpieces simultaneously, and in which two beam drivers are provided respectively on the left and right sides of the forging press. A three-directional motions, i.e., vertical motion (lift), advance and return (feed), and opening-and-closing motion (clamp) are combined and applied to two beams disposed for horizontally connecting horizontally the mentioned two beam drivers and placing them between upper and lower molds, whereby a workpiece clamped between clamping fingers attached inwardly to both beams are transferred to and placed at a required position.
The most important requirement for normally continuing the operation of such a transfer press is a perfect synchronization of actuation of the press with actuation of the transfer device which feeds a workpiece to each mold and, upon completing one forming step, moves therefrom to transfer the workpiece to the following mold. Without perfect synchronization, clamping fingers of the transfer device may be caught between a lowering press ram and the mold, thereby bringing about a serious problem relating to breakdown or damage. To overcome this problem, it has been conventionally employed that actuation of the transfer device is introduced from a drive mechanism of the transfer press itself, and synchronization is established through a transmission mechanism formed by combining mechanical elements such as a cam, a gear, a rack, etc. A lot of attempts to meet various kinds of problems occurring in the course of transmission have also heretofore proposed.
For example, Japanese Patent Publication (examined) No. Sho 55-32480 discloses a construction in which a pulse generator mounted on a main shaft of a press is combined with another pulse generator mounted on a rotary cam switch side coupled to the mentioned main shaft in such a manner as to generate pulses alternately, whereby an emergency stop is operated when no pulse is outputted from one pulse generator between pulses from the other pulse generator (i.e., when there is no alternative pulse generation). Japanese Patent Publication (unexamined) No. Sho 60-44137 discloses a construction in which a total value comprised of a value set by a feed setting digital switch and a predetermined value added thereto is compared with an actual value obtained from the rotational frequency of a motor in a comparator, and a stop signal is outputted when the actual value exceeds the mentioned total value. Japanese Patent Publication (unexamined) No. Hei 3-226325 discloses a construction in which a range finder for measuring a feed bar position at a specific stop point in a series of operations of the feed bar is disposed to monitor feed bar position. Japanese Utility Model Registration Publication (unexamined) No. Sho 61-63332 discloses a construction in the normal presence of a workpiece is preliminarily stored and is compared with the actually detected presence of the workpiece at every station.
In the mentioned conventional mechanisms, however, since the association or combination among cam disk, gear rack, etc. is fixed, stroke, timing and speed of the three-directional motion of the beams are feasible only under fixed values, and it is impossible to vary those factors depending upon size and type of workpiece, resulting in several practical use problems except in large size automatic forging presses in which workpiece variation is small.
That is, in most cases the operation of the transfer type of forging press popularly employed in the industry, workpieces to be forged are directed to the production of a wide variety but a small quantity. To meet such a practical need, it has been conventional to change the arrangment of molds or change the clamping fingers to be mounted on the beams, thereby performing a required variety of movements of the beams using the fixed transfer mechanism. Needless to say, such a conventional change of mold arrangement or clamping fingers results in considerable reduction in productivity of the entire forging press and, moreover, brings about rather troublesome maintenance. Furthermore, the mentioned combination is obliged to be mechanically complicated and results in a large-sized drive mechanism due to employment of a lot of stiff components and parts, occupying a large space on the left and right sides of the forging press.
In recent years, an idea has ben proposed and adopted for controlling the three-demensional motion of the feed bar by providing a servo motor as a drive mechanism independent of the drive mechanism of the press itself, whereby a technique has been developed, and in which various strokes and timing of operations are freely established by software in order to continue a complicated motion of the transfer mechanism. Thus it is said that most of the mentioned problems have been solved. In this recent technology, however, a direct association between operation of the press and that of the transfer device is not maintained and both of them are operated independently of each other. As a result, to achieve a perfect synchronization, more accurate control than those of the known arrangement is an essential requirement.
Japanese Patent Publication (unexamined) No. Sho 62-9731 proposes a technique for securing synchronization at the time of operating the transfer device through a servo mechanism independent of the driving of the press itself. According to this technique, a target position of a controlled member of the transfer press is decided according to an actual position of the press of the transfer press, and not only the controlled member is caused to follow the decided target position but also the actual position of the controlled member is compared with a critical allowance value decided according to the target position, whereby quick control is performed if the critical value is exceeded. Generally in known servo control, when any data indicating a target position of a motor is given, a control operation is basically performed in such a manner that data indicating an actual position of the motor fed back by encoder or the like follow the mentioned target position. On the other hand, in this known technique, a critical value of allowance is calculated and, when the actual position exceeds the allowed critical value or limit, any emergency means such as an alarm, stopper is operated.
FIG. 5(A) is a perspective view of this known arrangement showing the operation of a beam 4a operated by a servo motor 11a, and in which a rack 101 is mated with a pinion 102 and, accordingly, when driving the servo motor 11a connected to the pinion shaft, the beam 4a moves forward (advanding movement) or backward (returning movement). Since a position error is established to be within a predetermined allowance as shown in FIG. 5(B), and the allowable error is stored in a control unit, if occuring any positional error getting out of the predetermined allowance, an instruction signal is instantly outputted so as to give an alarm or instruct an emergency stop of the system operation, thereby the entire system is prevented from serious troble or accident.
However, what is recognized as a concept common to all of the mentioned known arrangements, including the mechanical synchronization system between the transfer press and the transfer device by mechanical association or combination of relative members and the electrical synchronization system utilizing a servo control, is directed just to an acknowledgment of the operational position of the transfer device relative to operational position of the transfer press. In other words, to a positional acknowledgement of a predetermined specific point of the beam and a malfunction detection thereof. It is sometimes a teaching in practical operation that mere detection of the mentioned two operational positions of a normal positional relation therebetween is not always sufficient control information to prevent all kinds of troubles, and that the timing of such detection itself is to late. Even when a specific position of a member of the transfer device such as a beam (feed bar) subject to a detection is detected to be under normal positional relation to the operational position of the press, it does not mean that all positions of peripheral members of the beam are covered by such detection. That is, members subject to the malfunction detection are quite limited. As a result, if any member not subject to detection should drop out or such member should get in the drive transmission system and is caught therein, serious trouble in the form of an accident can be caused thereby, which cannot be foreseen so long as there is no abnormality detection as to the position of the member subject to detection.
There may be a case in which, if any member of the transmission mechanism of the servo motor or driving unit should drop out, transmission of the following operation is suspended even when a member subject to detection is actually in a normal position and the servo motor is running at idle, thus any change in the normal position of the beam is not transmitted. As a result, serious trouble in the form of an accident of the transfer device may follow by interference with the actuating mold of the press.
There may arise another case in which, unless a certain quantity of lubricating oil is distributed to the operating parts, an excessive load is applied to the servo motor eventually resulting, in a short time period, in malfunction. Such a malfuntion cannot be forseen either by just detecting beam position.